Part of a typical known NFC system is shown schematically at 10 in FIG. 1. In the system of FIG, 1 an NFC reader 12 comprises a power amplifier 14 whose output is connected to input terminals of an antenna 16 by means of an amplifier filter having capacitors 18a, 18b which are connected in series between differential outputs of the power amplifier 14 and input terminals of the antenna 16.
An NFC tag 22 communicates with the reader 12 by means of an antenna 24, with the other components of the tag 22 being represented by a capacitor 26 and a resistor 28 connected in parallel with the antenna 24.
The power amplifier 14 is implemented as part of an integrated circuit (i.e. is an “on-chip” component), whilst the capacitors 18a, 18b are off-chip components (i.e. they are external to the integrated circuit containing the power amplifier 14) of fixed value.
It will be appreciated that the capacitors 18a, 18b and the antenna 16 form a series resonant circuit. The resonant frequency of this circuit is determined at least in part by the capacitance values of the capacitors 18a, 18b. For optimum transmission of data it is important that the resonant frequency of the series resonant circuit is equal to, or at least very close to, the frequency of the signal to be transmitted by the reader 12.
However, the capacitance values of the capacitors 18a, 18b are subject to manufacturing tolerances. These tolerances make it impossible to guarantee that the resonant frequency of the series resonant circuit will be equal to the frequency of the signal to be transmitted, and thus in NFC readers of the type shown in FIG. 1 transmission of data between the reader 12 and the tag 22 is rarely optimum.
This gives rise to two problems. If the signal to be transmitted by the reader 12 is generated by a voltage source the power transfer to the tag 22 may be reduced. If the signal to be transmitted by the reader 12 is generated by a current source the transmitted signal may be distorted if the input impedance of the antenna 16 is too high.
This second problem occurs during NFC operation when the coupling factor between the antenna 16 of the reader 12 and the antenna 24 of the tag 22 changes, and this also produces a change in the resonant frequency of the series resonant circuit and in the equivalent input series resistance of the reader 12. The signal received at the tag may then reach a peak amplitude when the coupling factor has not reached its peak value. Any reduction in the distance between the reader antenna 16 and the tag antenna 24 after this critical coupling point has been reached causes the amplitude of the input signal at the tag 22 to fall.